Reactor Theory (Reactor Operations)DOE-HDBK-1019/2-93REACTOR OPERATIONDecayHeatAbout 7 percent of the 200 MeV produced by an average fission is released at some time afterthe instant of fission. This energy comes from the decay of the fission products. When areactor is shut down, fission essentially ceases, but decay energy is still being produced. Theenergy produced after shutdown is referred to as decay heat. The amount of decay heatproduction after shutdown is directly influenced by the power history of the reactor prior toshutdown. A reactor operated at full power for 3 to 4 days prior to shutdown has much higherdecay heat generation than a reactor operated at low power for the same period. The decay heatproduced by a reactor shutdown from full power is initially equivalent to about 5 to 6% of thethermal rating of the reactor. This decay heat generation rate diminishes to less than 1%approximately one hour after shutdown. However, even at these low levels, the amount of heatgenerated requires the continued removal of heat for an appreciable time after shutdown. Decayheat is a long-term consideration and impacts spent fuel handling, reprocessing, wastemanagement, and reactor safety.SummaryThe important information in this chapter is summarized below.Reactor Operation SummaryAn installed neutron source, together with the subcritical multiplication process,may be needed to increase the neutron population to a level where it can bemonitored throughout the startup procedure.Reactivity balances, such as Estimated Critical Position calculations, typicallyconsider the basic reactivity of the core and the reactivity effects of temperature,direct xenon, and indirect xenon.A reactivity balance called an Estimated Critical Position is used to predict theposition of the control rods at which criticality will be achieved during a startup.To arrive at an ECP of the control rods, the basic reactivity, direct and indirectxenon reactivity, and temperature reactivity are added together to determine theamount of positive reactivity that must be added by withdrawing control rods toattain criticality. A graph of control rod worth versus rod position is used todetermine the estimated critical position.Rev. 0NP-04Page 33